Biodiesel, which is new prospect in oil and fat industry, are long-chain fatty acid ester substances generated by transesterification reaction of biological oil and fat feedstock. It is a novel renewable energy resource without pollution, and is referred to as biodiesel. The combustion properties of it are comparable to the traditional petroleum diesel. Since the harmful substances in the exhaust emitted from an engine in which biodiesel is combusted is 50% reduced when compared to traditional petroleum diesel, extensive attention has recently been drawn to the investigations and applications of biodiesel.
Currently, biodiesel is primarily produced by chemical processes, i.e. using animal/plant oil and fat together with some low carbon alcohols (methanol or ethanol) to conduct transesterification reaction under the catalysis of acidic or alkaline catalysts, so as to generate corresponding fatty acid methyl ester or ethyl ester. There exist the following inevitable disadvantages when using chemical processes to prepare biodiesel: (1) the free fatty acids and water in the oil and fat feedstock severely affect the progression of the reaction; (2) the poor solubility of methanol in oil and fat would easily result in the formation of emulsion, such that the subsequent processing procedure will become complicated; and (3) the amount of methanol used according to the requirements of the process greatly exceeds the molar ratio of the reaction, and the recovery of the excess methanol increases the energy consumption during the process.
A process using a biological enzyme for synthesizing biodiesel has the following advantages: mild reaction conditions, no pollutant emitted, and broad applicability for various oil and fat feedstock. Such process is in accordance with the green chemistry trend, and thus is getting increasing attention. However, in conventional lipase-catalyzing processes of converting oil and fat feedstock for the preparation of biodiesel, when the water content in the oil and fat feedstock is greater than 0.5% (based on the weight of the oil), the acid value of the biodiesel product after the reaction is generally higher than 0.5 mg KOH per gram of oil. This does not fulfill the acid value standard required for China and International biodiesel quality, and thus subsequent processing procedure involving complicated alkaline neutralization is required afterwards. Such subsequent processing procedure using alkaline neutralization to decrease the acid value affects the yield of the product, and brings about pollution problems.